The incidence of Alzheimer's disease (AD) is estimated to currently affect up to 4 million Americans and this number will certainly increase as our population continues to age. Approximately 3% of the population between ages 65 to 74 suffer from AD, and this value increases with increasing age. Since AD patients generally live 8 to 10 years after the disease is diagnosed, the financial burden AD imposes on our economy is estimated to exceed $100 billion and will continue to increase. Although there are a number of acetylcholinesterase inhibitors as approved drugs that can improve the symptoms of AD patients over the short term, no current therapy can retard disease progression. It is widely accepted that the oligomerization and subsequent deposition of amyloid β peptides (Aβ) is a major factor in AD. Thus considerable effort has been expended in the development of drugs that can selectively inhibit the β and γ secretases responsible for Aβ formation. To date no such drugs have reached the marketplace.
In recent years attention has been given to the peptidases that are involved in amyloid peptide (e.g., Aβ) clearance, such as neprilysin and insulysin. It has been shown that inhibition or deletion of these peptidases in rodent models leads to elevated Aβ, and that introduction of these peptidases into the brain of transgenic mice expressing human amyloid precursor protein (hAPP) can lead to a reduction in Aβ levels (Leissring M A, et al. (2003). Enhanced proteolysis of beta-amyloid in APP transgenic mice prevents plaque formation, secondary pathology, and a premature death. Neuron 40, 1087-1093). In addition expression of neprilysin in the brain can reduce the number of preformed amyloid plaques (Marr, R. A. et al. (2003) Neprilysin gene transfer reduces amyloid pathology in mouse models of Alzheimer's disease. J. Neurosci. 23:1992-1996). This is the first report that shows that neprilysin can actually be used to “dissolve” preformed plaques. Since neprilysin does not degrade aggregated Aβ, this demonstrates that the aggregated Aβ must be in a dynamic equilibrium with free Aβ or small Aβ oligomers. We have also conducted preliminary experiments designed to test the effect of neprilysin expression in the brain of an hAPP mouse model as a way to degrade Aβ and prevent amyloid deposits from forming. As shown in FIG. 3, we found that using the lentivirus-neprilysin construct to express neprilysin in the hippocampus of the J20 human hAPP transgenic mouse model of AD, virtually eliminated amyloid deposits in the 9-month old mouse. (Id.). In the lentivirus-NEP treated brain (FIG. 3B, right) there is a small amount of diffuse light staining material, indicative of amyloid peptide, as compared to the control lentivirus-GFP (FIG. 3A, right). (Id.).
More recently interest has emerged in targeting the clearance of Aβ peptides as a therapeutic approach, primarily through the use of antibodies to Aβ. This approach involves either immunizing with Aβ (Schenk D, et al. (1999) Immunization with amyloid-β attenuates Alzheimer-disease-like pathology in the PDAPP mouse. Nature 400:173-177; Janus C, et al. (2000) A peptide immunization reduces behavioral impairment and plaques in a model of Alzheimer's disease. Nature 408:979-982; Morgan D et al. (2000) Aβ peptide vaccination prevents memory loss in an animal model of Alzheimer's disease. Nature 408:982-985; Weiner H L, et al. (2000) Nasal administration of amyloid-β peptide decreases cerebral amyloid burden in a mouse model of Alzheimer's disease. Ann Neurol 48:567-579; Das P, et al. (2001) Reduced effectiveness of Aβ1-42 immunization in APP transgenic mice with significant amyloid deposition. Neurobiol Aging 22:721-727) or through the passive administration of Aβ antibodies (Bard F, et al. (2000) Peripherally administered antibodies against amyloid β-peptide enter the central nervous system and reduce pathology in a mouse model of Alzheimer disease. Nat Med 6:916-919; DeMattos R B, et al. (2001) Peripheral anti-Aβ antibody alters CNS and plasma Aβ clearance and decreases brain Aβ burden in a mouse model of Alzheimer's disease. Proc Natl Acad Sci USA 98:8850-8855; DeMattos R B, et al. (2002) Brain to plasma amyloid-β efflux: a measure of brain amyloid burden in a mouse model of Alzheimer's disease. Science 295:2264-2267). The promising results derived from Aβ immunization studies in mice led to clinical trials using Aβ immunization that initially appeared to produce beneficial results (Hock C, et al. (2003) Antibodies against beta-amyloid slow cognitive decline in Alzheimer's disease. Neuron. 38:547-54). However, several patients developed encephalitis (Greenberg S M, et al. (2003) Alzheimer disease's double-edged vaccine. Nat Med. 9:389-390) and the clinical trails were stopped. Consequently there exists a need to safely and effectively treat Alzheimer's disease.
One of the theories that immerged from the immunological studies was that passive immunization with Aβ antibodies resulted in an efflux of Aβ from the brain into the plasma producing a peripheral “sink effect”, DeMattos et al., 2001, 2002. It has been shown that peripheral administration of an anti-Aβ monoclonal antibody resulted in a rapid 1,000 fold increase in plasma Aβ and a marked reduction in Aβ deposition in the brain, DeMattos et al., 2001. Subsequently, it was shown that introduction of two Aβ-binding compounds, ganglioside GM1 and gelsolin, to bind plasma Aβ in hAPP transgenic mice, resulted in a lowering of brain Aβ levels by 50% or more (Matsuoka Y, et al. (2003) Novel therapeutic approach for the treatment of Alzheimer's disease by peripheral administration of agents with an affinity to beta-amyloid. J Neurosci. 23:29-33). In this study it was demonstrated that the lowering of brain Aβ by ganglioside GM1 was not due to ganglioside GM1 crossing the blood-brain barrier. It has also been shown that exogenous Aβ is rapidly transported from the CSF to plasma, exhibiting a half-time of ˜30 min (Ghersi-Egea J F, et al. (1996) Fate of cerebrospinal fluid-borne amyloid beta-peptide: rapid clearance into blood and appreciable accumulation by cerebral arteries. J Neurochem. 67:880-883; Shibata M, et al. (2000) Clearance of Alzheimer's amyloid-ss(1-40) peptide from brain by LDL receptor-related protein-1 at the blood-brain barrier. J Clin Invest. 106:1489-99). Yet another example is to use a soluble receptor for advanced glycation end products (RAGE) in the blood to bind Aβ (Deane R. et al. (2003) RAGE mediates amyloid-β peptide transport across the blood-brain barrier and accumulation in brain. Nat. Med. 9, 907-913).
The present invention comprises methods of lowering plasma Aβ levels as a way to lower brain Aβ using peripheral expression of amyloid peptide inactivating enzymes, like neprilysin, on hematopoietic cells. The methods of lowering plasma Aβ comprise i) peripheral expression of an amyloid peptide inactivating enzyme in hematopoietic stem cells comprising a viral vector to infect bone marrow stem cells with a neprilysin-expressing or other Aβ-degrading peptidase-expressing construct; ii) coupling of neprilysin or other Aβ degrading peptidases to hematopoietic cells; iii) modifying amyloid peptide inactivating enzymes such that the enzyme will bind to hematopoietic cells; or iv) use of liposomes or other agents to introduce neprilysin or other Aβ degrading peptidases into hematopoietic cells.